The family-friendly Cross Marin trail parallels lovely Lagunitas Creek (also called Paper Mill Creek). It consists of some paved sections (the northern end and where it travels through Samuel P. Taylor State Park) and hard-packed gravel or dirt for the remainder. Because the trail runs on the bed of the old North Pacific Coast Railroad (which ran from Sausalito up the coast to the Russian River) the grade is mostly flat and mellow.
Officially, the northern terminus of Cross Marin is the Platform Bridge Road and the southern end is the Inkwell Bridge area, but the trail can be used in either direction and in sections. You'll bike under second growth redwoods and along stretches of more open, oak-covered hillsides. There are opportunities for wading or swimming in Lagunitas Creek.
Be aware: one part of the Cross Marin trail (near the Samuel P. Taylor State Park entrance and campgrounds) is also referred to as the Riding and Hiking Trail on park signs. Also, this trail is extremely popular with horses, dogs, hikers, and kids- so be prepared to share.